What are closures and how do capture lists work?
Assesses fundamental understanding of Swift conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
A closure is a self-contained block of functionality that can be passed around and captures values from its surrounding context. Functions are a special case of closures.
let add = { (a: Int, b: Int) -> Int in a + b }
let names = ["Ada", "Grace"].sorted { $0 < $1 }
Syntax tools include trailing closure syntax and shorthand argument names such as $0 and $1. Capture lists control how references are captured: [weak self] or [unowned self] prevent retain cycles when a closure is stored by the object it references.
Closures are reference types, so capturing a variable captures the box that holds it. Escaping closures are annotated @escaping and outlive the function call; non-escaping is the default and allows compiler optimisations. Use closures for completion handlers, callbacks and functional pipelines like map and filter.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.